I am not familiar with OpenFOAM so I can only tell you how to fix the transition point numerically. Assuming that you know the region where the flow is laminar, you should define this region in term of x, y, z before executing the simulation. You will then need to modify your flow solver in such a way that the turbulent viscosity is set to zero in every cell within the predefined region. Downstream of this region, the turbulence model will force the flow to become turbulent and the transition point is approximately at the interface. Of course this only works if you already know the transition point.

hi tue, thank you for you quick answer and sorry for my english!
ok, i use openfoam.
i know that i can modify the solvers of the tutorials, and compile them. i want to use simplefoam.
now my question is: which file (for example a header file) may i modify to impose that before the point of transition, the nut is zero, and after this point the value of nut is different for zero??
thank very much

As I said before, I am not familiar with OpenFOAM so I cannot tell you where exactly you can make that modification. One way to do it is as follows: you should have a routine that compute an intermittent variable. This variable should be 0 in the laminar region that you prescribe and 1 elsewhere. Then you should look for the routine where the production of turbulence kinetic energy is computed (this production term is a source term so look for source terms). You can then simply multiply the intermittent variable to the production term. This means that there is no production of turbulence in the laminar region and thus the turbulent viscosity is null.

Implementing a transition model is an extension of the above procedure. You typically will need to implement transport equation(s) for the intermittent variable based on physical arguments and empiricism.

Does OpenFOAM have a laminar-turbulent transition model? Can't seem to find one, but I might have missed it somewhere. Is anyone working on this?

/Mads

There is a new RAS model in OpenFOAM (called as "kOmegaSSTLM") based on the Langtry-Menter Four Equation Transition Model. And one of the simpleFOAM tutorial T3A in OpenFOAM V1612 uses it, so you can refer it.